Thermal barrier coatings for components in high-temperature mechanical systems

ABSTRACT

An article that includes a substrate; a first layer including yttria and zirconia or hafnia, where the first layer has a columnar microstructure and includes predominately the zirconia or hafnia; a second layer on the first layer, the second layer including zirconia or hafnia, ytterbia, samaria, and at least one of lutetia, scandia, ceria, neodymia, europia, and gadolinia, where the second layer includes predominately zirconia or hafnia, and where the second layer has a columnar microstructure; and a third layer on the second layer, the third layer including zirconia or hafnia, ytterbia, samaria, and a rare earth oxide including at least one of lutetia, scandia, ceria, neodymia, europia, and gadolinia, where the third layer has a dense microstructure and has a lower porosity than the second layer.

This application claims the benefit of U.S. Provisional Application No.62/533,422 filed Jul. 17, 2017, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present disclosure relates to thermal barrier coatings.

BACKGROUND

Components of high-temperature mechanical systems, such as, for example,gas turbine engines, operate in severe environments. For example, thehigh-pressure turbine blades and vanes exposed to hot gases incommercial aeronautical engines typically experience exterior surfacetemperatures of about 1000° C., with short-term peaks as high as 1100°C. Example components of high-temperature mechanical systems may includea Ni-based or Co-based super alloy substrate or a ceramic or ceramicmatrix composite substrate.

Economic and environmental concerns such as the desire for improvedefficiency and reduced emissions, continue to drive the development ofadvanced gas turbine engines with higher inlet temperatures. Substratesof high-temperature mechanical systems may be coated with a thermalbarrier coating (TBC) to reduce the substrate temperatures in order tomeet the operational limits of the component.

SUMMARY

In some examples, the disclosure describes an article that includes asubstrate; a first layer including a first base oxide including zirconiaor hafnia and a first rare earth oxide including yttria, where the firstlayer has a columnar microstructure, where the first layer includespredominately the first base oxide; a second layer on the first layer,the second layer including a second base oxide including zirconia orhafnia, a second rare earth oxide including ytterbia, a third rare earthoxide including samaria, and a fourth rare earth oxide including atleast one of lutetia, scandia, ceria, neodymia, europia, and gadolinia,where the second layer includes predominately the second base oxide, andwhere the second layer has a columnar microstructure; and a third layeron the second layer, the third layer including a third base oxideincluding zirconia or hafnia, the second rare earth oxide, the thirdrare earth oxide, and a fifth rare earth oxide including at least one oflutetia, scandia, ceria, neodymia, europia, and gadolinia, where thethird layer has a dense microstructure and has a lower porosity than thesecond layer.

In some examples, the disclosure describes a method including depositinga first plurality of particles on a substrate to form a first layer,where the first plurality of particles includes a first base oxideincluding zirconia or hafnia and a first rare earth oxide includingyttria, and where the first layer includes a columnar microstructure;depositing a second plurality of particles on the first layer using asuspension plasma spray technique to form a second layer, where thesecond plurality of particles includes a second base oxide includingzirconia or hafnia, a second rare earth oxide including ytterbia, athird rare earth oxide including samaria, and a fourth rare earth oxideincluding at least one of lutetia, scandia, ceria, neodymia, europia,and gadolinia, where the second layer includes a columnarmicrostructure; and depositing a third plurality of particles using asuspension plasma spray technique to form a third layer on the secondlayer, where the third plurality of particles includes a third baseoxide including zirconia or hafnia, the second rare earth oxide, thethird rare earth oxide, and a fifth rare earth oxide including at leastone of lutetia, scandia, ceria, neodymia, europia, and gadolinia, wherethe third layer has a dense microstructure and has a lower porosity thanthe second layer.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic diagram illustrating an example system forforming an article that includes a multi-layer TBC on a substrate usinga suspension plasma spray technique.

FIG. 1B is an enlarged cross-sectional view of the example article fromFIG. 1A that includes a multi-layer TBC formed on substrate.

FIG. 2 is a cross-sectional diagram of an example article that includesa multi-layer TBC deposited on a bond coat and a substrate using asuspension thermal spray technique.

FIG. 3 is a cross-sectional diagram of another example article thatincludes a multi-layer TBC deposited on a substrate using a suspensionthermal spray technique.

FIG. 4 is a flow diagram illustrating an example technique fordepositing a layer on a substrate using suspension plasma spray.

FIG. 5 is a flow diagram illustrating an example technique fordepositing a multi-layer TBC on a substrate using a suspension plasmaspray technique.

DETAILED DESCRIPTION

The disclosure describes articles including a multi-layer thermalbarrier coating (TBC) formed using suspension plasma spray techniquesand techniques of forming the same. In some examples, TBCs may have alow thermal conductivity to reduce the transfer of thermal energy fromthe high-temperature gases to the substrate. However, erosion andcontamination can reduce the life of TBCs, which may make the TBC lesseffective at protecting the underlying substrate. Erosion and/orcontamination may occur when deleterious environmental species, such as,for example, calcium magnesium aluminum silicate (CMAS), penetrate theTBC. The presence of a deleterious environmental species in the TBC mayweaken or degrade the TBC layers, causing damage to an underlyingsubstrate due to stresses imposed on the TBC during thermal cyclingwithin the high-temperature operational environments. For example, CMASmay migrate into the layers of the TBC, reducing the insulativeproperties of the layer and/or physically stressing the TBC layerleading to spallation. Additionally, or alternatively, the CMAS maymigrate through the TBC into underlying layers or to the underlyingsubstrate, leading to unwanted side reactions.

A TBC with a dense microstructure may help prevent some of thedeleterious environmental species from migrating into the TBC andcausing degradation of the TBC, or may help prevent deleteriousenvironmental species from migrating to other layers or the substrateand causing additional degradation of the article. However, a TBC withonly a dense microstructure may be subject to increased in-plane strainduring thermal cycling, such as, for example, in a high-temperature gasturbine engine. A TBC with only a dense microstructure also may exhibitincreased thermal conductivity making the TBC less effective.

In some examples, improved thermal cycling performance of the TBC may beobtained with a TBC having a predominately columnar microstructure. Acolumnar microstructure may include columns of the coating materialextending from the surface of a substrate with elongated intercolumnarvoids that have a crystallographic texture. A columnar microstructuremay allow for the TBC to have improved in-plane strain tolerance and adecreased thermal conductivity. However, a columnar microstructure maybe less durable due to the increased porosity. Traditional depositiontechniques, such as, for example, electron beam-physical vapordeposition (EB-PVD), may be capable of producing such columnarmicrostructures. However, EB-PVD may be complex, expensive, limited bythe number of manufacturing sites with EB-PVD technology, and may havepoor process efficiency, such as, for example, less than 10% efficiencyin deposition of raw materials. Further, EB-PVD processes may not allowfor deposition of multi-layer coatings including more than onemicrostructure or phase constituents.

As described herein, one or more layers of the multi-layer TBC may bedeposited on a substrate using a suspension plasma spray technique. Thesuspension plasma spray technique may be used to form a multi-layer TBChaving at least two different microstructures, such as, for example, atleast one layer with a dense microstructure and at least one layer witha columnar microstructure to provide synergistic properties to theresultant TBC. For example, the multi-layer structure of the TBC mayhelp prevent deleterious environmental species from migrating into theTBC or to the substrate, as well as provide improved thermal cyclingperformance of the TBC.

In some examples, the suspension plasma spray techniques describedherein may be used to deposit a relatively small particle coatingmaterial (e.g., average particle size less than about 1 μm to about 25μm, or less than about 1 μm to about 10 μm) in order to obtain aselected coating microstructure (e.g., dense or columnar). As usedherein, “average particle diameter,” “average diameter,” or “particlesize” may be an equivalent mean diameter (e.g., if the particles are notspherical) of a given particle size distribution.

Such particle sizes may be insufficient for deposition by traditionalthermal spray techniques, such as plasma spray techniques, whichtypically utilize a minimal particle size of about 30-60 μm to avoidagglomeration or fouling of the thermal spray device. Additionally, thesuspension plasma spray techniques may provide a more efficient andcost-effective way of producing the multi-layer TBC compared to vapordeposition techniques (e.g., EB-PVD).

FIG. 1A is a schematic diagram illustrating an example system 10 forforming an article 40 that includes a multi-layer TBC 18 on a substrate16 using a suspension plasma spray technique. System 10 includes achamber 12 that encloses a stage 14 configured to receive substrate 16,a suspension source 22, a plasma spray device 20 that receives asuspension 26 (e.g., coating material 28 suspended in carrier 30) fromsuspension source 22, and a computing device 24 configured to controlthe feed of suspension 26 from suspension source 22 to thermal spraydevice 20 and the subsequent deposition of coating material 28 to formmulti-layer TBC 18 on substrate 16.

In some examples, article 40 may include a component of a gas turbineengine. For example, article 40 may include a part that forms a portionof a flow path structure, a seal segment, a blade track, an airfoil, ablade, a vane, a combustion chamber liner, or another portion of the gasturbine engine. FIG. 1B is an enlarged cross-sectional view of theexample article 40 from FIG. 1A that includes multi-layer TBC 18 onsubstrate 16 using system 10. As used herein, “formed on” and “on” meansa layer or coating that is formed on top of another layer or coating andencompasses both a first layer or coating formed immediately adjacent asecond layer or coating and a first layer or coating formed on top of asecond layer or coating with one or more intermediate layers or coatingspresent between the first and second layers or coatings. In contrast,“formed directly on” and “directly on” denote a layer or coating that isformed immediately adjacent to another layer or coating, i.e., there areno intermediate layers or coatings. In some examples, as shown in FIG.1B, multi-layer TBC 18 may be directly on substrate 16.

Substrate 16 may include a material suitable for use in ahigh-temperature environment. In some examples, substrate 16 includes asuper alloy including, for example, an alloy based on Ni, Co, Ni/Fe, orthe like. In examples where substrate 16 includes a super alloymaterial, substrate 16 may also include one or more additives such astitanium (Ti), cobalt (Co), or aluminum (Al), which beneficially affectthe mechanical properties of substrate 16 including, for example,toughness, hardness, temperature stability, corrosion resistance,oxidation resistance, or the like.

In some examples, substrate 16 may include a ceramic or a ceramic matrixcomposite (CMC). Suitable ceramic materials, may include, for example, asilicon-containing ceramic, such as silica (SiO₂), silicon carbide(SiC); silicon nitride (Si₃N₄); alumina (Al₂O₃); an aluminosilicate; atransition metal carbide (e.g., WC, Mo₂C, TiC); a silicide (e.g., MoSi₂,NbSi₂, TiSi₂); combinations thereof; or the like. In some examples inwhich substrate 16 includes a ceramic, the ceramic may be substantiallyhomogeneous.

In examples in which substrate 16 includes a CMC, substrate 16 mayinclude a matrix material and a reinforcement material. The matrixmaterial may include, for example, silicon metal or a ceramic material,such as silicon carbide (SiC), silicon nitride (Si₃N₄), analuminosilicate, silica (SiO₂), a transition metal carbide or silicide(e.g., WC, Mo₂C, TiC, MoSi₂, NbSi₂, TiSi₂), or other ceramics describedherein. The CMC may further include a continuous or discontinuousreinforcement material. For example, the reinforcement material mayinclude discontinuous whiskers, platelets, fibers, or particulates.Additionally, or alternatively, the reinforcement material may include acontinuous monofilament or multifilament two-dimensional orthree-dimensional weave. In some examples, the reinforcement materialmay include carbon (C), silicon carbide (SiC), silicon nitride (Si₃N₄),an aluminosilicate, silica (SiO₂), a transition metal carbide orsilicide (e.g. WC, Mo₂C, TiC, MoSi₂, NbSi₂, TiSi₂), another ceramicmaterial described herein, or the like.

In some examples, the composition of the reinforcement material is thesame as the composition of the matrix material. For example, a matrixmaterial including silicon carbide may surround a reinforcement materialincluding silicon carbide whiskers or fibers. In other examples, thereinforcement material includes a different composition than thecomposition of the matrix material, such as aluminosilicate fibers in analumina matrix, or the like. In some examples, substrate 16 thatincludes a CMC comprising a reinforcement material of silicon carbidefibers embedded in a matrix material of silicon carbide. In someexamples, substrate 16 includes a SiC—SiC CMC.

Multi-layer TBC 18 may be deposited on substrate 16 using the suspensionplasma spray techniques of the present disclosure. Multi-layer TBC 18may reduce the transfer of thermal energy from high-temperature gases tosubstrate 16; help prevent deleterious environmental species (e.g.,CMAS) from migrating into the layers of TBC 18, any optional underlyinglayers, or substrate 16; provide erosion resistance; improve thermalcycling performance of article 40; or combinations thereof.

Multi-layer TBC 18 includes a first layer 42, a second layer 44, and athird layer 46. Each layer of multi-layer TBC 18 may contribute toproperties of multi-layer TBC 18, and each layer may be selectedindependently to provide similar or different properties to multi-layerTBC 18. For example, first layer 42 may provide improved thermal cyclingperformance, second layer 44 may provide a low thermal conductivity, andthird layer 46 may improve erosion resistance and/or CMAS resistance.

First layer 42 includes a first base oxide of either zirconia or hafniaand a first rare earth oxide of yttria deposited in a columnarmicrostructure on substrate 16. For example, first layer 42 may includeyttria-stabilized zirconia or hafnia, that includes predominately (e.g.,the main component or a majority) of the first base oxide zirconia(ZrO₂) or hafnia (HfO₂) mixed with a minority amount of yttria (Y₂O₃).In some examples, the first base oxide may consist of zirconia. The useof the terms “first,” “second,” “third,” etc. oxide is used in anordinal sense to identify and distinguish among the different oxidecomponents of the various layers rather than in the cardinal sense tolimit or imply the total number of oxides that may be present within arespective layer.

In some examples, first layer 42 may consist essentially of zirconia andyttria. As used herein, to “consist essentially of” means to consist ofthe listed element(s) or compound(s), while allowing the inclusion ofimpurities present in small amounts such that the impurities do nosubstantially affect the properties of the listed element or compound.For example, the purification of many rare earth elements may bedifficult, and thus the nominal rare earth element may include smallamounts of other rare earth elements. This mixture is intended to becovered by the language “consists essentially of.” In some examples,first layer 42 may consist essentially of yttria-stabilized-zirconia,which includes about 92 weight percent (wt. %) to about 94 wt. % of thebase oxide zirconia stabilized by about 6 wt. % to about 8 wt. % of therare earth oxide yttria.

In some examples, having first layer 42 consist essentially of zirconiaand yttria may improve the layer's thermal cycling resistance (e.g., along thermal cycling life), and/or adhesion to underlying substrate 16or an optional bond coat. For example, first layer 42 consistingessentially of zirconia and yttria may reduce the coefficient of thermalexpansion of the layer such that it is more comparable to that ofsubstrate 16. Additionally, or alternatively, the overall high purity offirst layer 42 (e.g., compared to the purity of second layer 44 whichmay include additional oxides) may reduce the chance of side reactionsor coefficient of thermal expansion mismatches within the layer toprovide better long-term adhesion between first layer 42 and substrate16.

First layer 42 may have a coefficient of thermal expansion that liesbetween that of substrate 16 and second layer 44. In this way, thecoefficient of thermal expansion mismatch may be reduced due to firstlayer 42 acting as an intermediate or gradient layer between substrate16 and second layer 44. In turn, first layer 42 may reduce stress due tothermal expansion between substrate 16 and second layer 44 to improvethe working life of article 40.

In some examples, first layer 42 may be deposited on substrate 16 usingthe suspension plasma spray techniques of the present disclosure. Thesuspension plasma spray techniques may allow the resultant first layer42 to have a substantially columnar microstructure that providesimproved thermal cycling performance by reducing the in-plane strainexerted between substrate 16 and first layer 42 during thermal cyclingin comparison to a comparable layer that does not possess a columnarmicrostructure.

As described further below, the columnar microstructure of first layer42 may be obtained using system 10 to spray deposit a coating material28 that includes very fine particles (e.g., average particle size lessthan about 1 μm) of the base oxide (e.g., zirconia) and rare earth oxideyttria. As used herein, “very fine particles” is intended to describeparticles with an average particle diameter of less than about 1 μm.During the suspension plasma spray process, the very fine particles ofcoating material 28 may be carried by the plasma stream of plasma spraydevice 20 to be deposited on substrate 16. Due to the small particlesize, coating material 28 is more likely to be deflected within theplasma stream as the stream contacts the surface of substrate 16. Thedeflection causes the very fine particles of coating material 28 todeposit on substrate 16 at angles other than normal to the surface ofsubstrate 16. This process allows coating material 28 to be depositedwith the formation of columns within the microstructure, which mayotherwise not be possible with larger particle sizes (e.g., greater than1 μm). For example, coating material 28 may follow trajectories of theplasma stream as the stream contacts the surface of substrate 16 anddeflects horizontally along the surface of substrate 16. In turn, ascoating material 28 is deposited, the deposits form asperities creatingshadows downstream of the trajectory of coating material 28 in theplasma stream. In some such examples, coating material 28 may notdeposit in at least some of the shadows, resulting in the formation theintercolumnar voids of the columnar microstructure. The columnarmicrostructure of first layer 42 may provide increased in-plane straintolerance and improved thermal cycling resistance, resulting in betteradhesion properties and a more robust article 40.

In other examples, first layer 42 may be deposited using techniquesother than the suspension plasma spray techniques described hereinincluding, for example, traditional thermal spraying, including, airplasma spraying, high velocity oxy-fuel (HVOF) spraying, low vaporplasma spraying; physical vapor deposition (PVD), including EB-PVD,directed vapor deposition (DVD), and cathodic arc deposition; chemicalvapor deposition (CVD); slurry process deposition; sol-gel processdeposition; electrophoretic deposition; or the like. In the case inwhich first layer 42 is deposited using an alternative depositiontechnique, first layer 42 may still include a substantially columnarmicrostructure. However, compared to some alternative techniques (e.g.,EB-PVD) the suspension plasma spray techniques described herein maydemonstrate a higher conversion yield of the raw materials into thecoating layer (e.g., an efficiency of more than about 50% as compared toan efficiency of about 10% associated with EB-PVD).

In some examples, first layer 42 may be a relatively thin layer. Forexample, first layer 42 may be between about 0.0005 inches and about0.003 inches (e.g., between about 10 μm and about 80 μm). Even at theserelatively small thicknesses, first layer 42 may contribute to thermalcycling performance of multi-layer TBC 18.

Second layer 44 may include a second base oxide of zirconia or hafniaand at least one rare earth oxide, such as, for example, oxides of Lu,Yb, Tm, Er, Ho, Dy, Gd, Tb, Eu, Sm, Pm, Nd, Pr, Ce, La, Y, Sc, andcombinations thereof, on first layer 42 with a columnar microstructure.Second layer 44 may include predominately (e.g., the main component or amajority) the base oxide zirconia or hafnia mixed with a minorityamounts of the at least one rare earth oxide.

In some examples, second layer 44 may include the second base oxide anda second rare earth oxide including ytterbia, a third rare earth oxideincluding samaria, and a fourth rare earth oxide including at least oneof lutetia, scandia, ceria, neodymia, europia, and gadolinia. In someexamples, the fourth rare earth oxide may include gadolinia such thatthe second layer 44 may include the second base oxide (e.g., zirconia),ytterbia, samaria, and gadolinia deposited on first layer 42 with acolumnar microstructure. Second layer 44 may include predominately(e.g., the main component or a majority) the second base oxide (e.g.,zirconia) mixed with a minority amounts of ytterbia, gadolinia, andsamaria. The below description of second layer 44 is primarily describedwith respect to the second base oxide including zirconia and the second,third, and fourth rare earth oxides including ytterbia, gadolinia, andsamaria, however in other examples, other rare earth oxides may be usedand/or hafnia may be used as the second base oxide.

In some examples, the composition (e.g., zirconia, ytterbia, gadolinia,and samaria) and the columnar microstructure of second layer 44 mayprovide improved thermal insulation and protection to substrate 16 fromhigh temperatures, e.g., high-temperature of the turbine gas compared toother coating compositions or microstructures. For example, duringoperation of article 40 in a high temperature environment, heat istransferred through multi-layer TBC 18 through conduction and radiation.The inclusion of one or more rare earth oxides, such as ytterbia,gadolinia, and samaria within a layer of predominately zirconia may helpdecrease the thermal conductivity of second layer 44. While not wishingto be bound by any specific theory, the inclusion of ytterbia,gadolinia, and samaria in second layer 44 may reduce thermalconductivity through one or more mechanisms, including phonon scatteringdue to point defects and grain boundaries in the zirconia crystallattice due to the rare earth oxides, reduction of sintering, andporosity.

The composition of second layer 44 may be selected to provide a desiredphase constitution. Accessible phase constitutions include tetragonalprime (t′), cubic, and compound RE₂O₃—ZrO₂ or RE₂O₃—HfO₂ (where RE is arare earth element) phase constitutions measured using x-raydiffraction. Second layer 44 may include tetragonal prime (t′), cubic,or compound phase constitutions or combinations thereof.

For example, to achieve a compound phase constitution, a layer mayinclude about 20 mol. % to about 40 mol. % ytterbia, about 10 mol. % toabout 20 mol. % gadolinia, about 10 mol. % to about 20 mol. % samaria,and the balance the respective base oxide (e.g., zirconia or hafnia) andany impurities present.

To achieve a cubic phase constitution, a layer may include about 3 mol.% to about 10 mol. % ytterbia, about 1 mol. % to about 5 mol. %gadolinia, about 1 mol. % to about 5 mol. % samaria, and the balance therespective base oxide (e.g., about 80 mol. % to about 95 mol. % zirconiaor hafnia) and any impurities present.

To achieve a tetragonal prime phase constitution, a layer may includeabout 1 mol. % to about 5 mol. % ytterbia, about 0.1 mol. % to about 3mol. % gadolinia, and about 0.1 mol. % to about 3 mol. % samaria, andthe respective base oxide (about 89 mol. % to about 98.8 mol. % zirconiaor hafnia) and any impurities present.

In some examples, second layer 44 may include or include a majority of acubic phase constitution (e.g., the majority of second layer 44 consistsof a cubic phase constitution). In some examples, second layer 44 mayconsist essentially of a cubic phase constitution. The cubic phaseconstitution may provide second layer 44 with a lower thermalconductivity than a layer having a similar composition, but with atetragonal prime or a compound phase constitution.

In some examples, second layer 44 may include ytterbia in aconcentration of between about 3 mol. % and about 10 mol. %, gadoliniain a concentration between about 1 mol. % and about 5 mol. %, samaria ina concentration between about 1 mol. % and about 5 mol. %, and thebalance zirconia and any impurities present in a cubic phaseconstitution. In some examples, second layer 44 may include ytterbia ina concentration of between about 3.5 mol. % and about 4.5 mol. % (e.g.,about 4 mol. %), gadolinia in a concentration between about 2.5 mol. %and about 3.5 mol. % (e.g., about 3 mol. %), samaria in a concentrationbetween about 2.5 mol. % and about 3.5 mol. % (e.g., about 4 mol. %),and the balance zirconia (e.g., about 88.5 mol. % to about 91.5 mol. %)and any impurities present in a cubic phase constitution.

In some examples, the inclusion of ytterbia, gadolinia, and samaria insecond layer 44 may also provide second layer 44 with increasedresistance to CMAS degradation compared by yttria-stabilized zirconia,reduce the thermal conductivity of second layer 44, or both. Althoughthe composition of second layer 44 is described with respect tozirconia, ytterbia, gadolinia, and samaria, one or more of the zirconia,ytterbia, gadolinia, and samaria may be replaced by one or more ofhafnia or a rare earth oxide, such as, for example, oxides of Lu, Yb,Tm, Er, Ho, Dy, Gd, Tb, Eu, Sm, Pm, Nd, Pr, Ce, La, Y, or Sc.

As with first layer 42, the suspension plasma spray techniques describedherein may be used to deposit second layer 44 on first layer 42. Forexample, system 10 may be used to deposit a very fine particle coatingmaterial 28 (e.g., average particle size less than about 1 μm) havingthe compositional makeup of second layer 44 to form second layer 44 witha columnar microstructure. As described above, coating material 28 withvery fine particle sizes may be used to generate the columnarmicrostructure which may otherwise not be obtained using traditionalthermal spray techniques. The use of very fine particles with thesuspension plasma spray techniques as described herein may result insecond layer 44 including a substantially columnar microstructure thatprovides improved thermal cycling performance and thermal insulativeproperties in comparison to other layers that do not possess a columnarmicrostructure.

In some examples, second layer 44 may have a thickness of between about0.001 inches and about 0.03 inches (e.g., between about 25 μm and about7650 μm). For example, second layer 44 may be between about 0.004 inchesand about 0.015 inches (e.g., between about 100 μm and about 380 μm).

Multi-layer TBC 18 also includes third layer 46, which may exhibit arelatively dense microstructure. The relatively dense microstructure mayreduce or substantially prevent exposure of substrate 16 to deleteriousenvironmental species (e.g., CMAS), prevent deterioration and erosion ofmulti-layer TBC 18, and increase the service life of substrate 16.

Third layer 46 may include a third base oxide of zirconia or hafnia andat least one rare earth oxide, such as, for example, oxides of Lu, Yb,Tm, Er, Ho, Dy, Gd, Tb, Eu, Sm, Pm, Nd, Pr, Ce, La, Y, Sc, andcombinations thereof, on second layer 44 with a dense microstructure.Third layer 46 may include predominately (e.g., the main component or amajority) the third base oxide of zirconia or hafnia mixed with aminority amounts of the at least one rare earth oxide.

In some examples, third layer 46 may include the third base oxide andthe second rare earth oxide including ytterbia, the third rare earthoxide including samaria, and a fifth rare earth oxide including at leastone of lutetia, scandia, ceria, neodymia, europia, and gadolinia. Insome examples, the fifth rare earth oxide may include gadolinia suchthat third layer 46 may include predominately (e.g., the main componentor a majority) the third base oxide (e.g., zirconia) mixed with aminority amounts of ytterbia, gadolinia, and samaria on second layer 44in a dense microstructure. The below description of third layer 44 isprimarily described with respect to the layer including zirconia and therare earth oxides ytterbia, gadolinia, and samaria, however in otherexamples, other rare earth oxides may be used and/or hafnia may be usedas the third base oxide.

As used herein, a “dense microstructure” may be characterized by a layerwith a relatively low resultant volume porosity (e.g., a porosity ofless than about 5 percent by volume (vol. %)). In other examples, thirdlayer 46 may have a porosity greater than about 5 vol. %, such as forexample a porosity of less than about 20 vol. %, such as less than about15 vol. %, or less than about 10 vol. %. In some examples, second layer44 may have first porosity, and third layer 46 may have a secondporosity, and the second porosity of third layer 46 may be less than thefirst porosity of second layer 44. The porosity of deposited third layer46 may be measured as a percentage of pore volume divided by totalvolume of the layer, and may be measured using optical microscopy ormercury porosimetry. In some examples, the porosity of third layer 46may be measured using ASTM B328-94. The relatively low level of porositymay reduce the migration of deleterious elements (e.g., CMAS) throughthird layer 46 that may otherwise damage or degrade substrate 16, otherlayers included in multi-layer TBC 18, or other layers within article40. Additionally, or alternatively, the composition of third layer 46and the relatively low porosity may improve the durability of the layerand article 40. The relatively low porosity of third layer 46 may alsoimprove the erosion resistance of third layer 46.

The composition of third layer 46 may be selected to provide one or moredesired phase constitutions, as described above with respect to secondlayer 44. In some examples, third layer 46 may include or include amajority of a tetragonal prime phase constitution. In some examples,third layer 46 may include ytterbia in a concentration of between about1 mol. % and about 5 mol. %, gadolinia in a concentration between about0.1 mol. % and about 3 mol. %, samaria in a concentration between about0.1 mol. % and about 3 mol. %, and the balance zirconia (e.g., about 89mol. % to about 98.8 mol. %) and any impurities present in the phaseconstitution. In some examples, third layer 46 may include ytterbia in aconcentration of between about 2 mol. % and about 4 mol. % (e.g., about2.5 mol. %), gadolinia in a concentration between about 0.1 mol. % andabout 2 mol. % (e.g., about 1 mol. %), samaria in a concentrationbetween about 0.1 mol. % and about 1 mol. % (e.g., about 0.5 mol. %),and the balance zirconia and any impurities present in a tetragonalprime phase constitution.

In some examples, the zirconia, ytterbia, gadolinia, and samaria presentin the third layer 46 may consist essentially of a tetragonal primephase constitution. Although the composition of third layer 46 isdescribed with respect to zirconia, ytterbia, gadolinia, and samaria,one or more of the zirconia, ytterbia, gadolinia, and samaria may bereplaced by one or more of hafnia or a rare earth oxide, such as, forexample, oxides of Lu, Yb, Tm, Er, Ho, Dy, Gd, Tb, Eu, Sm, Pm, Nd, Pr,Ce, La, Y, or Sc.

In some examples, a layer including a tetragonal prime phaseconstitution may have improved thermal cycling resistance and/ordurability in comparison to a layer including a cubic phaseconstitution, but generally exhibits a higher thermal conductivity thana comparable layer including a cubic phase constitution. Thus, byforming TBC 18 with second layer 44 having a columnar microstructure andsubstantially cubic phase constitution of zirconia, ytterbia, gadolinia,and samaria and third layer 46 with a dense microstructure andsubstantially tetragonal prime phase constitution of zirconia, ytterbia,gadolinia, and samaria, the two layers may provide multi-layer TBC 18with low thermal conductivity, improved thermal cycling resistance, andimproved overall durability.

In some examples, third layer 46 may be deposited with a densemicrostructure using the suspension plasma spray techniques describedherein. For example, by controlling one or more of the depositionparameters of the suspension plasma spray techniques, coating material28 may be deposited as relatively dense microstructure with lowporosity. One parameter that may affect the resultant microstructure ofthe deposited layer is the particle size of coating material 28. Forexample, coating material 28 including a fine particle (e.g., betweenabout 1 μm and about 25 μm) may result in third layer 46 with a densemicrostructure. As used herein, “fine particle” is intended to describeparticles with an average particle diameter between about 1 μm and about25 μm. The fine particles of coating material 28 (e.g., in comparison toother deposition techniques) may permit a more compressed arrangement ofthe deposited particles resulting in third layer 46 with a reducedporosity, reduced pore size, higher density, or combinations thereof.The increased density of third layer 46 may help prevent exposure of thesurface of a substrate to deleterious environmental species, preventdeterioration and erosion of multi-layer TBC 18, and increase theservice life of substrate 16.

In some examples, the size of the pores that are present in third layer46 may be smaller than pores generated using other thermal spraytechniques. For example, due to the particle size associated withtraditional plasma spray techniques (e.g., particle diameters on theorder of about 30-60 μm), the resultant pores produced between thedeposited particles will remain relatively large due to the geometricsize and shape of the deposited particles. Because the suspension plasmaspray techniques described herein can be used to deposit relativelysmall size particles (e.g., particle diameters less than about 25 μm),the pores between the deposited particles may likewise be reduced insize. The reduced pore size may result in the pores between depositedparticles to be less likely to be interconnected within the thickness ofthird layer 46. In some examples, pores of third layer 46 may be on theorder of about 1 μm to about 10 μm. The smaller pore size of third layer46 may reduce migration of deleterious compounds, e.g., CMAS, throughthird layer 46. Additionally, or alternatively, the smaller pore sizemay provide phonon scattering, increased difficulty of heat transferthough third layer 46, and/or a decreased thermal conductivity of thirdlayer 46.

Additionally, or alternatively, the suspension plasma spray techniquesdescribed herein may help reduce the overall production cost and timefor forming multi-layer TBC 18. For example, using traditionaltechniques, multi-layer TBC 18 may have to be deposited using multipletechniques, such as, for example, conventional plasma spraying andEB-PVD, in order to obtain multi-layer TBC 18 with more than onemicrostructure, e.g., second layer 44 with a columnar microstructure andthird layer 46 with a dense microstructure. Using the suspension plasmaspray techniques of the present disclosure, process parameters may beadjusted to influence the microstructure of the resultant layer ofmulti-layer TBC 18.

In some examples, third layer 46 may have a thickness of between about0.001 inches and about 0.005 inches (e.g., between about 25 μm and about130 μm).

In some examples, third layer 46 may further include alumina (Al₂O₃).The presence of alumina in third layer 46 may improve the durability andtoughness of third layer 46. Additionally, or alternatively, thepresence of alumina in third layer 46 may provide enhanced erosion andcontamination resistance of multi-layer TBC 18 compared to some TBCsthat do not include a layer including alumina. For example, including ofalumina may reduce a reaction rate with alumina components in CMAS.

In examples in which third layer 46 includes alumina, the layer mayinclude at least two distinct phase constitutions including a firstphase including the third base oxide (e.g., zirconia), the second rareearth oxide (e.g., ytterbia), the third rare earth oxide (e.g.,samaria), and the fifth rare earth oxide (e.g., gadolinia) (e.g., atetragonal prime phase constitution) and a second phase includingalumina. The presence of more than one phase may help enhance the creepstrength of third layer 46 compared to a single-phase layer, which inturn, may increase the durability and useful life of third layer 46 andmulti-layer TBC 18.

In some examples, the predominate phase (e.g., present at more than 50vol. %) of the third layer 46 may include the third base oxide (e.g.,zirconia), the second rare earth oxide (e.g., ytterbia), the third rareearth oxide (e.g., samaria), and the fifth rare earth oxide (e.g.,gadolinia). Depending on the composition of third layer 46, the aluminamay be present as a second phase dispersed within the first phase. Forexample, the first phase may be a substantially continuous throughoutthird layer 46 (e.g., the first phase material remains connectedthroughout third layer 46) with discrete second phase regions of aluminaincluded within the substantially continuous first phase.

In some examples, third layer 46 including alumina may include aboundary region between the first phase including the third base oxide(e.g., zirconia), the second rare earth oxide (e.g., ytterbia), thethird rare earth oxide (e.g., samaria), and the fifth rare earth oxide(e.g., gadolinia) and the second phase including the alumina. Theboundary region between the first and second phases may include areaction product from a reaction between the oxides of the first phaseand the alumina of the second phase or may include a different crystalstructure where the alumina alloys with, e.g., the zirconia, ytterbia,gadolinia, and samaria. Alternatively, or in addition, the optionalalumina of third layer 46 may be alloyed throughout the phase includingthe third base oxide (e.g., zirconia), the second rare earth oxide(e.g., ytterbia), the third rare earth oxide (e.g., samaria), and thefifth rare earth oxide (e.g., gadolinia).

In some examples, the first phase may include a tetragonal prime phaseconstitution with ytterbia in a concentration between about 2 mol. % andabout 4 mol. % (e.g., about 2.5 mol. %), gadolinia in a concentrationbetween about 0.1 mol. % and about 2 mol. % (e.g., about 1 mol. %),samaria in a concentration between about 0.1 mol. % and about 1 mol. %(e.g., about 0.5 mol. %), and the and the balance the third base oxide(e.g., zirconia) and any impurities present. The second phase of thirdlayer 46 may include or consist essentially of alumina. Third layer 46may include between about 10 mol. % and about 50 mol. % alumina. Forexample, third layer 46 may include between about 10 mol. % and about 50mol. % alumina, between about 10 mol. % and about 30 mol. % alumina, orbetween about 10 mol. % and about 20 mol. % alumina. The two phases maybe deposited as third layer 46 using the suspension plasma spraydeposition techniques described herein by, for example, pre-mixingparticles of the alumina phase and pre-alloyed tetragonal prime phaseconstitutions together in suspension 26.

Returning to FIG. 1A, system 10 may be used to apply one or more layersof multi-layer TBC 18 to substrate 16 using a suspension plasma spraytechnique. Chamber 12 may substantially enclose (e.g., enclose or nearlyenclose) stage 14 that receives substrate 16, and plasma spray device20. In some examples, stage 14 may be configured to selectively positionand restrain substrate 16 in place relative to plasma spray device 20during formation of multi-layer TBC 18. For example, stage 14 may betranslatable and/or rotatable along at least one axis to positionsubstrate 16 relative to plasma spray device 20 to facilitate theapplication of multi-layer TBC 18 on substrate 16 via plasma spraydevice 20.

System 10 also includes suspension source 22 configured to deliver asuspension 26 including a coating material 28 (e.g., the solid materialsthat form one of the layers of multi-layer TBC 18) and a carrier 30 toplasma spray device 20 or a plume generated by plasma spray device 20.In some examples, suspension source 22 may include a nozzle or otherapparatus within chamber 12 for introducing suspension 26 to plasmaspray device 20 or a plume generated by plasma spray device 20.Suspension source 22 may be communicatively coupled to computing device24, such that computing device 24 may control suspension source 22(e.g., opening or closing a valve, positioning suspension source 22,controlling a flow rate of suspension 26 from suspension source 22 toplasma spray device 20, or the like).

Coating material 28 may include a particle form of the respectivematerials used to form first layer 42, second layer 44, and third layer46 of multi-layer TBC 18. For example, coating material 28 may includezirconia and yttria to deposit first layer 42; zirconia, ytterbia,gadolinia, and samaria to deposit second layer 44; or zirconia,ytterbia, gadolinia, samaria, and optionally alumina, to deposit thirdlayer 46, as described above. Coating material 28 may be in the form ofparticles to facilitate softening or vaporization of coating material 28by a heated plume created by plasma spray device 20. In some examples,coating material 28 may include separate coating materials for eachrespective layer of first layer 42, second layer 44, and third layer 46.

In some examples, coating material 28 may include a single particletype, e.g., a pre-alloyed particle with the desired composition and/orphase constitution. The single particle type may allow for a uniformdisbursement and control the composition and/or phase constitution ofthe resultant layer of multi-layer TBC 18. The pre-alloyed particles mayinclude a desired phase constitution for the layer to be deposited,e.g., cubic or tetragonal prime phase constitutions. In other examples,coating material 28 may include discrete particles, e.g., distinctparticles of each of the base oxide, rare earth oxides, and alumina(where used) combined to make up the composition of the respective layerof multi-layer TBC 18. The particle materials may be mechanicallypremixed within suspension 26 prior to deposition. Due to the relativelysmall particle size used in the suspension plasma spray techniques, thediscrete particles may intimately mix during the deposition process toform the desired phase constitution.

In some examples, coating material 28 may have a very fine particlesize, which may result in deposition of a layer with a columnarmicrostructure. As described above, the very fine particle diametersizes may allow for vaporization of at least some of the particles andmay allow for coating material 28 to be deflected within the plasmastream, causing coating material 28 to deposit on substrate 16 at anglesother than normal to the surface of substrate 16 and resulting in alayer with a columnar microstructure. In some examples, the particlesize of coating material 28 for creating the columnar microstructure maydefine an average particle diameter between about 0.01 μm and about 1μm, between about 0.01 μm and about 0.5 μm, or between about 0.01 μm andabout 0.05 μm.

In other examples, coating material 28 may be deposited as a layerincluding a dense microstructure (e.g., third layer 46). In someexamples, the average particle diameter for producing a layer with adense microstructure may still remain relatively small (e.g., an averageparticle diameter less than about 25 μm, less than about 10 μm, or lessthan about 1 μm) compared to particle sizes used with traditional plasmaspray techniques. In some examples, the particles sizes may be similarto the particles sizes used to form a columnar microstructure but mayresult in a dense microstructure by modifying the deposition parameters,such as, for example, the spraying distance and/or power, of system 10.For example, decreasing the spraying distance, decreasing the suspensionfeed rate, and/or increasing the power may result in a densemicrostructure even when relatively small particles are used. In someexamples, the dense microstructure obtained by the suspension plasmaspray techniques described herein may allow for a layer of multi-layerTBC 18 to exhibit a decreased overall porosity and resultant higherdensity compared to a comparable layer deposited using traditionalthermal spray techniques.

Suspension 26 also includes carrier 30 that acts as a carrier fluid andallows small particles (e.g., less than about 25 μm or less than about10 μm) of coating material 28 to be used without agglomeration of theparticles prior to deposition. In some examples, carrier 30 may be awater-based or alcohol-based solvent. Examples of suitable materials forcarrier 30 may include, for example, water, ethanol, methanol, isopropylalcohol, or the like.

Coating material 28 may be added to carrier 30 to form suspension 26. Insome examples, suspension 26 may include may include about 1 vol. % toabout 30 vol. % solid loading of coating material 28 in carrier 30. Insome examples, coating material 28 may be added to carrier 30 to formsuspension 26 with a desired viscosity, stability of the colloidalsuspension, e.g., flocculation, heat capacity, or any other parameter tofit the needs of system 10. In some examples, suspension 26 may includea combustible liquid that may undergo an exothermic reaction uponspraying.

In some examples, suspension 26 may further include one or more deliveryaids (e.g., additives that do not form multi-layer TBC 18 but aid in thedelivery or deposition of coating material 28 within carrier 30).Examples of delivery aids may include one or more dispersants orsurfactants. In some examples, a surfactant may help improve thestability and dispersion of the colloidal suspension. In some examples,system 10 may further include one or more mixers in order to maintainsuspension 26, e.g., maintain coating material 28 suspended in carrier20.

Plasma spray device 20 may include a plasma spray gun including acathode and an anode separated by a plasma gas channel. As the plasmagas flows through the plasma gas channel, a voltage may be appliedbetween the cathode and anode to cause the plasma gas to form a plasma.In some such examples, suspension 26 may be injected inside plasma spraydevice 20 such that the suspension flows through part of the plasma gaschannel. In other examples, suspension 26 may be introduced to a plumeof the plasma external to plasma spray device 20. Upon introduction tothe plasma gas, carrier 30 in suspension 26 may evaporate allowingcoating material 28 to be heat softened or vaporized followed by thesubsequent deposition of coating material 28 on substrate 16 in the formof a layer of multi-layer TBC 18.

System 10 also includes computing device 24. Computing device 24 mayinclude, for example, a desktop computer, a laptop computer, aworkstation, a server, a mainframe, a cloud computing system, or thelike. Computing device 24 may include or may be one or more processors,such as one or more digital signal processors (DSPs), general purposemicroprocessors, application specific integrated circuits (ASICs), fieldprogrammable logic arrays (FPGAs), or other equivalent integrated ordiscrete logic circuitry. Accordingly, the term “processor,” as usedherein may refer to any of the foregoing structure or any otherstructure suitable for implementation of the techniques describedherein. In addition, in some examples, the functionality of computingdevice 24 may be provided within dedicated hardware and/or softwaremodules.

Computing device 24 may be configured to control operation of system 10,including, for example, stage 14, suspension source 22, and/or plasmaspray device 20. For example, computing device 24 may be configured tocontrol operation of stage 14, suspension source 22, and/or plasma spraydevice 20 to position substrate 16 relative to suspension source 22and/or plasma spray device 20. In such examples, computing device 24 maycontrol suspension source 22 and plasma spray device 20 to maneuver theposition of substrate 16 relative to plasma spray device 20 tofacilitate the deposition of multi-layer TBC 18.

Computing device 24 may be communicatively coupled to at least one ofstage 14, suspension source 22, and plasma spray device 20 usingrespective communication connections. Such connections may be wirelessor wired connections.

In some examples, article 40 may also include one or more intermediatelayers (e.g., a bond coat) positioned between TBC 18 and substrate 16.For example, FIG. 2 is a cross-sectional diagram of an example article60 that includes a multi-layer TBC 18 deposited on a bond coat 52 and asubstrate 16 using the suspension plasma spray techniques describedherein. Multi-layer TBC 18 may be the same or substantially the same asmulti-layer TBC 18 described with respect to FIG. 1B apart from anydifference noted below.

Bond coat 52 may be deposited on or deposited directly on substrate 16to promote adhesion between substrate 16 and one or more additionallayers deposited on bond coat 52, including, for example, multi-layerTBC 18 (e.g., first layer 42). In examples in which substrate 16includes a superalloy, bond coat 52 may include an alloy, such as aMCrAlY alloy (where M is Ni, Co, or NiCo), a β-NiAl nickel aluminidealloy (either unmodified or modified by Pt, Cr, Hf, Zr, Y, Si, andcombinations thereof), a γ-Ni+γ′-Ni₃Al nickel aluminide alloy (eitherunmodified or modified by Pt, Cr, Hf, Zr, Y, Si, and combinationsthereof), or the like. In some examples, bond coat 52 may include Pt.

In other examples, bond coat 52 may include ceramics or other materialsthat are compatible with substrate 16 that includes a ceramic or a CMC.For example, bond coat 52 may include mullite (aluminum silicate,Al₆Si₂O₁₃), silica, silicides, silicon, or the like. Bond coat 52 mayfurther include other ceramics, such as rare earth silicates includinglutetium (Lu) silicates, ytterbium (Yb) silicates, thulium (Tm)silicates, erbium (Er) silicates, holmium (Ho) silicates, dysprosium(Dy) silicates, gadolinium (Gd) silicates, terbium (Tb) silicates,europium (Eu) silicates, samarium (Sm) silicates, promethium (Pm)silicates, neodymium (Nd) silicates, praseodymium (Pr) silicates, cerium(Ce) silicates, lanthanum (La) silicates, yttrium (Y) silicates,scandium (Sc) silicates, or the like.

Bond coat 52 may be selected based on a number of considerations,including the chemical composition and phase constitution of multi-layerTBC 18 (e.g., first layer 42) and substrate 16. For example, whensubstrate 16 includes a superalloy with a γ-Ni+γ′-Ni₃Al phaseconstitution, bond coat 52 may include a γ-Ni+γ′-Ni₃Al phaseconstitution to better match the coefficient of thermal expansion ofsubstrate 16, and therefore increase the mechanical stability (adhesion)of bond coat 52 to substrate 16. Alternatively, when substrate 16includes a CMC, bond coat 52 may include silicon and/or a ceramic, suchas, for example, mullite or a rare earth silicate.

In some examples, bond coat 52 may include multiple layers. In some suchexamples, the different layers of bond coat 52 may perform separatefunctions. For example, in some examples in which substrate 16 is a CMCincluding silicon carbide, bond coat 52 may include a first layer ofsilicon deposited on substrate 16, followed by a second layer includingmullite or a rare earth silicate. The silicon layer may provide betterbonding to substrate 16, while the ceramic layer may prevent water vaporfrom reaching the silicon layer and/or provide better coefficient ofthermal expansion mating with the layers of TBC 18.

In some examples, bond coat 52 may be deposited using a suspensionplasma spray technique, e.g., as described herein. In other examples,bond coat 52 may be deposited using other techniques including, forexample, traditional thermal spraying, including, air plasma spraying,HVOF spraying, low vapor plasma spraying; PVD, including EB-PVD, DVD,and cathodic arc deposition; CVD; slurry process deposition; sol-gelprocess deposition; electrophoretic deposition; or the like.

Bond coat 52 may define any thickness adequate to promote adherence ofan additional layer to substrate 16. For example, bond coat 52 may havea thickness of less than about 0.008 inches (e.g., less than about 200μm).

FIG. 2 shows an example article 50 that includes substrate 16, andmulti-layer TBC 18 deposited on bond coat 52. In some examples, article50 may or may not include all of the layers shown in FIG. 2, or article50 may have one or more additional layers included, such as, forexample, an EBC layer, an abradable coating, an outer CMAS-resistantlayer, or the like.

In some examples, the third layer 46 of TBC 18 may be separated into twodifferent layers in order to further tailor the properties of thirdlayer 46. For example, FIG. 3 is a cross-sectional diagram of anotherexample article 80 that includes a multi-layer TBC 62 deposited onsubstrate 16 using a suspension thermal spray technique as describedherein. Multi-layer TBC 62 includes first layer 42, second layer 44,third layer 64, and fourth layer 66. First layer 42 and second layer 44may be the same as first layer 42 and second layer 44 described abovewith respect to FIG. 1B, the details of which will not be repeated here.

The composition of third layer 64 may be the same or substantially thesame as third layer 46 from FIG. 1B that does not include added alumina.For example, third layer 64 may include predominately (e.g., the maincomponent or a majority) the third based oxide (e.g., zirconia) mixedwith a minority amounts of the second rare earth oxide (e.g., ytterbia),the third rare earth oxide (e.g., samaria), and the fifth rare earthoxide (e.g., gadolinia) deposited on second layer 44 as a densemicrostructure. In some examples, third layer 64 may include atetragonal prime phase constitution with about 2.5 mol. % ytterbia,about 1 mol. % gadolina, about 0.5 mol. % samaria, and the balancezirconia and any impurities present.

Third layer 64 may include a dense microstructure formed using thesuspension plasma spray techniques described herein. The increaseddensity of third layer 64 may help prevent exposure of substrate 16 todeleterious environmental species, prevent deterioration of multi-layerTBC 62, and increase the service life of substrate 16.

In some examples, third layer 64 may have a thickness of between about0.0005 inches to about 0.005 inches (e.g., between about 10 μm to about130 μm).

TBC 62 also includes fourth layer 66 on third layer 64. The compositionof fourth layer 66 may be the same or substantially the same as thirdlayer 46 from FIG. 1B that includes a phase constitution of alumina. Forexample, fourth layer 66 may include a fourth based oxide of zirconia orhafnia, the second rare earth oxide including ytterbia, the third rareearth oxide including samaria, and a sixth rare earth oxide including atleast one of lutetia, scandia, ceria, neodymia, europia, and gadolinia(e.g., gadolinia) collectively forming a first phase and alumina forminga second phase. In some examples, the first phase of fourth layer 46 mayinclude a tetragonal prime phase constitution with ytterbia in aconcentration between about 2 mol. % and about 4 mol. % (e.g., about 2.5mol. %), gadolinia in a concentration between about 0.1 mol. % and about2 mol. % (e.g., about 1 mol. %), samaria in a concentration betweenabout 0.1 mol. % and about 1 mol. % (e.g., about 0.5 mol. %), and thebalance zirconia, and the second phase constitution of alumina. Fourthlayer 66 may include between about 10 mol. % and about 50 mol. % ofalumina based on the total layer. As described above, the presence ofalumina may help increase the durability of fourth layer 66 and helpprotect multi-layer TBC 62 from erosion due to deleterious species, suchas, for example, carbon and sand.

In some examples, fourth layer 66 may include a dense microstructure.The increased density of fourth layer 66 may help prevent exposure ofsubstrate 16 to deleterious environmental species, prevent deteriorationand erosion of multi-layer TBC 62, and increase the service life ofsubstrate 16. In other examples, fourth layer 66 may include a columnarmicrostructure.

In some examples, fourth layer 66 may have a thickness between about0.0005 inches and about 0.003 inches (e.g., between about 10 μm andabout 80 μm).

Fourth layer 66 may be deposited using the suspension plasma spraytechniques described herein. In other examples, fourth layer 66 may bedeposited using other techniques including, for example, traditionalthermal spraying, including, air plasma spraying, HVOF spraying, lowvapor plasma spraying; PVD, including EB-PVD, DVD, and cathodic arcdeposition; CVD; slurry process deposition; sol-gel process deposition;electrophoretic deposition; or the like to deposit fourth layer 66including a dense microstructure or a columnar microstructure.

FIG. 4 is a flow diagram illustrating an example technique 70 fordepositing a layer on a substrate using suspension plasma spraying. Thetechnique of FIG. 4 is described with respect to system 10 of FIG. 1Aand second and third layers 44, 46 of article 40 of FIG. 1B for ease ofdescription only. However, the suspension plasma spray techniques may beused to form other layers/articles of article 40 and the layers of TBC18 of FIG. 1B may be formed using suspension plasma spray techniquesother than those described in FIG. 4 or using other systems than thoseshown in FIG. 1A.

Technique 70 of FIG. 4 includes forming suspension 26 including coatingmaterial 28 and carrier 30 (72). Coating material 28 and carrier 30 maybe substantially the same as those described above with respect tosystem 10 of FIG. 1A and article 40 of FIG. 1B. For example, withrespect to forming second and third layers 44, 46, coating material 28may include a respective base oxide (e.g., zirconia), the second rareearth oxide (e.g., ytterbia), the third rare earth oxide (e.g.,samaria), and an additional rare earth oxide including at least one oflutetia, scandia, ceria, neodymia, europia, and gadoliniazirconia,ytterbia, gadolinia (e.g., gadolinia) mixed with a carrier 30. Asdescribed above, the respective amounts of each oxide may be tailored toformulate a cubic phase constitution (e.g., second layer 44), atetragonal prime phase constitution (e.g., third layer 46), or acombination thereof depending on the desired properties of the layer.The particles may be pre-alloyed (e.g., particles of cubic or tetragonalprime phase constitutions of zirconia, ytterbia, gadolinia, and samaria)or provided as discrete particles of the different oxides mechanicallymixed in suspension 26 in the desired compositional amounts.

In some examples, coating material 28 may have a fine particle size tofacilitate melt softening or vaporization of coating material 28 by aheated plume (e.g., plasma) of plasma spray device 20. The averageparticle size of coating material 28 may be less than about 25 μm orless than about 10 μm, and in some cases, less than about 1 μm,depending on the desired microstructure of the resultant layer.

Altering the size of the particles of coating material 28, the sprayingdistance, power of plasma spray device 20, injection position, size ofinjection nozzle, surface speed, advance rate, target temperature,suspension feed rate, carrier 30 or combinations thereof may affect themicrostructure, thickness, phase(s), and porosity of a deposited layer.For example, where a columnar microstructure is desired (e.g., secondlayer 44) the average particle diameter may be less than about 1 μm suchas between about 0.01 μm and about 1 μm, between about 0.01 μm and about0.5 μm, or between about 0.01 μm and about 0.05 μm. The very fineparticles of coating material 28 may be deposited using the suspensionplasma spray techniques described herein to form a layer with a columnarmicrostructure. For example, as one non-limiting example, the very fineparticles may be applied using a relatively high spraying distance, arelatively high suspension feed rate, a relatively low power, orcombinations thereof to obtain the columnar microstructure; howeverother parameters may also be used to obtain the columnar microstructure.

In other examples where a dense microstructure is preferred (e.g., thirdlayer 46), the fine particles making up coating material 28 may have anaverage particle diameter between about 1 μm and about 25 μm, betweenabout 1 μm and about 20 μm, between about 1 μm and about 10 μm, orbetween about 1 μm and about 5 μm. As one non-limiting example, the veryfine particles may be applied using a relatively low spraying distance,a relatively low suspension feed rate, a relatively high power, orcombinations thereof to obtain the dense microstructure; however otherparameters may also be used to obtain the dense microstructure.Additionally, because the particles remain relatively small compared tothe particle sizes utilized in other plasma spray techniques (e.g., >30μm), the relatively small particle sizes of coating material 28 mayresult in the layer having a reduced pore size compared to a layerdeposited using a traditional thermal spray technique, as the smallparticles will pack closer together.

Coating material 28 may be added to carrier 30 to form suspension 26. Insome examples, suspension 26 may include may include about 1 to about 30vol. % solid loading of coating material 28 in carrier 30. Suspension 26may further include delivery aids such as a dispersant or a surfactant.The dispersant or surfactant may prevent coating material 28 fromagglomerating in carrier 30 and maintain the suspension of coatingmaterial 28 in carrier 30.

Once suspension 26 is formed, suspension 26 may be introduced into aheated plume formed by plasma spray device 20 (74). For example,computing device 24 may control suspension source 22 to provide acontrolled amount or rate of suspension 26 into the heated plume formedby plasma spray device 20.

Suspension 26 may be stored or supplied to plasma spray device 20 usingsuspension source 22. Computing device 24 may control suspension source22 to introduce a controlled amount of suspension 26 into the heatedplume formed by plasma spray device 20.

The temperature of the heated plume may, in some examples, be aboveabout 6000 K, which may result in evaporation of substantially all(e.g., all or nearly all) of carrier 30. The evaporation of carrier 30may leave substantially only coating material 28 in the heated plume.The high temperature of the heated plume may also result in meltsoftening or vaporization of coating material 28.

Technique 70 further includes directing coating material 28 towardsubstrate 16 using the heated plume (76). For example, computing device24 may control a position of plasma spray device 20, stage 14, or both,to cause the heated plume to be directed at a selected location ofsubstrate 16 to result in coating material 28 being deposited at theselected location. The heated plume may carry coating material 28 towardsubstrate 16, where coating material 28 deposits in a layer (e.g.,second layer 44 or third layer 46) on substrate 16 (78).

In some examples, system 10 may include an inert gas source (not shownin FIG. 1A), and the inert gas source may supply an inert gas shroud tocoating material 28 in the heated plume during deposition of coatingmaterial 28 on substrate 16. The inert gas may surround the heated plumeas the heated plume exits plasma spray device 20. The inert gas shroudmay reduce in-air oxidation of coating material 28. In-air oxidation maycause the resulting multi-layer TBC 18 to have reduced density, cohesivestrength, bond strength, or the like. The inert gas used for the inertgas shroud may include Ar, N₂, or the like.

In some examples, while directing coating material 28 toward substrate16 using the heated plume (76), computing device 24 may control plasmaspray device 20, stage 14, or both to move plasma spray device 20 andsubstrate 16 relative to each other. For example, computing device 24may be configured to control plasma spray device 20 to scan the heatedplume relative to substrate 16. This may cause the cylinder-shapedheated plume that includes coating material 28 to move relative tosubstrate 16, and may form a layer of multi-layer TBC 18 over thesurfaces of substrate 16 scanned with the heated plume.

FIG. 5 is a flow diagram illustrating an example technique 80 fordepositing a multi-layer TBC on a substrate using a suspension plasmaspray technique as described herein. The technique of FIG. 5 isdescribed with respect to articles 40, 50, and 60 of FIGS. 1B-3 for easeof description only. A person having ordinary skill in the art willrecognize and appreciate that the technique of FIG. 5 may be used toform articles other than those of FIGS. 1B-3, and the articles of FIGS.1B-3 may be formed using other suspension plasma spray techniques thanthose described in FIG. 5.

Technique 80 includes depositing an optional bond coat 52 on substrate16 (82), depositing first layer 42 on substrate 16 (84), depositingsecond layer 44 on first layer 42 using plasma spray device 20 (86), anddepositing third layer 46 on second layer 44 using plasma spray device20 (88).

The composition of bond coat 52 may be tailored depending on thecomposition of underlying substrate 16 in order to improve adhesionbetween multi-layer TBC 18 and substrate 16 and may be substantially thesame as described above with respect to FIG. 2. For example, wheresubstrate 16 includes a super alloy material, bond coat 52 may includean alloy, such as a MCrAlY alloy or may include Pt. In other exampleswhere substrate 16 includes a ceramic or CMC, bond coat 52 may includeceramics.

In some examples, bond coat 52 may be deposited on substrate 16 using asuspension plasma spray technique, e.g., technique 70 of FIG. 4. Inother examples, the bond coat may be applied using other techniquesincluding, for example, traditional thermal spraying, including, airplasma spraying, HVOF spraying, low vapor plasma spraying; PVD,including EB-PVD, DVD, and cathodic arc deposition; CVD; slurry processdeposition; sol-gel process deposition; electrophoretic deposition; orthe like.

Technique 80 includes also depositing first layer 42 of multi-layer TBC18 on substrate 16 (84). First layer 42 may be substantially the same asfirst layer 42 described above with respect to article 40 of FIG. 1. Forexample, first layer 42 may include zirconia and yttria deposited in acolumnar microstructure to provide multi-layer TBC 18 with enhancedthermal cycling resistance and bonding to underlying substrate 16.

First layer 42 may be deposited using a first plurality of particlesusing the suspension plasma spray techniques of the present disclosure,e.g., technique 70 of FIG. 1. The first plurality of particles mayinclude a single particle type, e.g., a pre-alloyed zirconia and yttriaparticles, or may include discrete particles of zirconia and yttria. Insome examples, the first plurality of particles may have an averagediameter less than about 1 μm and may be deposited in a predominatelycolumnar microstructure.

Technique 80 further includes depositing second layer 44 in a columnarmicrostructure on first layer 42 (86). Second layer 44 may besubstantially the same as second layer 44 described with respect to FIG.1B. For example, second layer 44 may include zirconia, ytterbia,gadolinia, and samaria deposited in a columnar microstructure to providemulti-layer TBC 18 with a low thermal conductivity.

Second layer 44 may be deposited using a second plurality of particlesusing the suspension plasma spray techniques described herein, e.g.,technique 70 of FIG. 4. In some examples, the second plurality ofparticles may include a single particle type, e.g., a pre-alloyedzirconia, ytterbia, gadolinia, and samaria particles. The pre-alloyedparticles may include or substantially include pre-alloyed particles ina desired phase constitution. For example, the second plurality ofparticles may include zirconia, ytterbia, gadolinia, and samariaincluding or substantially including a cubic phase constitution. Inother examples, the second plurality of particles may include discreteparticles, e.g., distinct particles of each of zirconia, ytterbia,gadolinia, and samaria combined to make up the composition of secondlayer 44 with the desired phase constitution. In some examples,depositing second layer 44 as a cubic phase constitution may providesecond layer 44 with a lower thermal conductivity than a layer having asimilar composition with a tetragonal prime or a compound phaseconstitution.

In some examples, the second plurality of particles may define anaverage particle size less than about 1 μm which may aid in depositingsecond layer 44 as a columnar microstructure. The columnarmicrostructure of second layer 44 may provide improved thermal cyclingperformance in comparison to layers that do not possess a columnarmicrostructure.

Technique 80 also includes depositing third layer 46, in a densemicrostructure on second layer 44 (88). Third layer 46 may besubstantially the same as third layer 46 described with respect to FIG.1B. For example, third layer 46 may include zirconia, ytterbia,gadolinia, samaria, and optionally, alumina, in a dense microstructureto help prevent exposure of substrate 16 to deleterious environmentalspecies, prevent deterioration and erosion of multi-layer TBC 18, andincrease the durability and service life of substrate 16.

Third layer 46 may be deposited using a third plurality of particlesusing the suspension plasma spray techniques described herein, e.g.,technique 70 of FIG. 4. In some examples, the third plurality ofparticles may include a single particle type, e.g., a pre-alloyedzirconia, ytterbia, gadolinia, and samaria particles. The pre-alloyedparticles may include or substantially include pre-alloyed particles ina desired phase constitution. For example, the third plurality ofparticles may include zirconia, ytterbia, gadolinia, and samariaincluding or substantially including a tetragonal prime phaseconstitution. In other examples, the third plurality of particles mayinclude discrete particles, e.g., distinct particles of each ofzirconia, ytterbia, gadolinia, and samaria combined to make up thecomposition of third layer 46 with the desired phase constitution. Insome examples, depositing third layer 46 as a tetragonal prime phaseconstitution may provide third layer 46 with improved thermal cyclingresistance than a layer having a similar composition with a cubic or acompound phase constitution.

In some examples, the third plurality of particles may define an averageparticle size between about 1 μm and about 25 μm which may aid indepositing third layer 46 with a dense microstructure. The densemicrostructure of third layer 46 may provide a decreased overallporosity and resultant higher density in comparison to layers that donot possess a dense microstructure.

Technique 80 also includes optionally depositing one or more additionallayers on multi-layer TBC 18 (90). In some examples, the additionallayer be the same or substantially the same as fourth layer 66 of FIG.3. For example, fourth layer 66 may include a first phase constitutionof the fourth base oxide (e.g., zirconia), the second rare earth oxide(e.g., ytterbia), the third rare earth oxide (e.g., samaria), and thesixth rare earth oxide (e.g., gadolinia) collectively, for example, in atetragonal prime phase constitution and a second phase of alumina asdescribed with respect to FIG. 3, where the alumina phase makes up about10% to about 50% of the layer. In some examples, fourth layer 66 may bedeposited over a third layer 64 that does not include alumina. Thepresence of alumina in fourth layer 66 may increase the overalldurability of article 60 compared to an outer layer on article 60 thatdoes not include alumina.

Fourth layer 66 may be deposited using a fourth plurality of particles(e.g., particles of alumina mixed with particles of the first phaseconstitution of zirconia, ytterbia, gadolinia, samaria) and a suspensionplasma spray technique, e.g., technique 70 from FIG. 4. In otherexamples, fourth layer 66 may be applied using other techniquesincluding, for example, traditional thermal spraying, including, airplasma spraying, HVOF spraying, low vapor plasma spraying; PVD,including EB-PVD, DVD, and cathodic arc deposition; CVD; slurry processdeposition; sol-gel process deposition; electrophoretic deposition; orthe like.

In some examples, fourth layer 66 may be deposited as a densemicrostructure, a columnar microstructure, or a porous microstructure toprovide desired properties. For example, fourth layer 66 may bedeposited in a columnar microstructure to provide enhanced thermalcycling resistance, or fourth layer 66 may be deposited in a densemicrostructure to provide enhanced erosion resistance and durability.

In some examples, one or more additional layers may be deposited onthird layer 46 or fourth layer 66 including, for example, otherprotective or functional layers, such as, for example, an EBC layer, anabradable coating, a CMAS-resistant layer, or the like.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. An article comprising: a substrate; a first layercomprising a first base oxide comprising zirconia or hafnia and a firstrare earth oxide comprising yttria, wherein the first layer has acolumnar microstructure, wherein the first layer comprises predominatelythe first base oxide; a second layer on the first layer, the secondlayer comprising a second base oxide comprising zirconia or hafnia, asecond rare earth oxide comprising ytterbia, a third rare earth oxidecomprising samaria, and a fourth rare earth oxide comprising at leastone of lutetia, scandia, ceria, neodymia, europia, and gadolinia,wherein the second layer comprises predominately the second base oxide,and wherein the second layer has a columnar microstructure; and a thirdlayer on the second layer, the third layer comprising a third base oxidecomprising zirconia or hafnia, the second rare earth oxide, the thirdrare earth oxide, and a fifth rare earth oxide comprising at least oneof lutetia, scandia, ceria, neodymia, europia, and gadolinia, whereinthe third layer has a dense microstructure and has a lower porosity thanthe second layer.
 2. The article of claim 1, further comprising a bondcoat between the substrate and the first layer.
 3. The article of claim1, wherein the second layer comprises a cubic phase constitution, andwherein the third layer comprises a tetragonal prime phase constitution.4. The article of claim 3, wherein the second layer comprises betweenabout 3 mol. % and about 10 mol. % ytterbia, between about 1 mol. % andabout 5 mol. % gadolinia, between about 1 mol. % and about 5 mol. %samaria, and a balance zirconia, and wherein the third layer comprisesbetween about 1 mol. % and about 5 mol. % ytterbia, between about 0.1mol. % and about 3 mol. % gadolinia, and between about 0.1 mol. % andabout 3 mol. % samaria, and a balance zirconia.
 5. The article of claim1, wherein the third layer further comprises alumina in a separatephase.
 6. The article of claim 1, further comprising a fourth layercomprising a fourth base oxide comprising zirconia or hafnia, alumina,the second rare earth oxide, the third rare earth oxide, and a sixthrare earth oxide comprising at least one of lutetia, scandia, ceria,neodymia, europia, and gadolinia, wherein the fourth layer has a densemicrostructure and has a lower porosity than the second layer.
 7. Thearticle of claim 6, wherein the fourth layer comprises a tetragonalprime phase comprising the zirconia, ytterbia, gadolinia, samaria; andan alumina phase.
 8. The article of claim 7, wherein at least one of thefirst layer, the second layer, the third layer, or the fourth layer isdeposited using a suspension plasma spray technique.
 9. A methodcomprising: depositing a first plurality of particles on a substrate toform a first layer, wherein the first plurality of particles comprises afirst base oxide comprising zirconia or hafnia and a first rare earthoxide comprising yttria, and wherein the first layer comprises acolumnar microstructure; depositing a second plurality of particles onthe first layer using a suspension plasma spray technique to form asecond layer, wherein the second plurality of particles comprises asecond base oxide comprising zirconia or hafnia, a second rare earthoxide comprising ytterbia, a third rare earth oxide comprising samaria,and a fourth rare earth oxide comprising at least one of lutetia,scandia, ceria, neodymia, europia, and gadolinia, wherein the secondlayer comprises a columnar microstructure; and depositing a thirdplurality of particles using a suspension plasma spray technique to forma third layer on the second layer, wherein the third plurality ofparticles comprises a third base oxide comprising zirconia or hafnia,the second rare earth oxide, the third rare earth oxide, and a fifthrare earth oxide comprising at least one of lutetia, scandia, ceria,neodymia, europia, and gadolinia, wherein the third layer has a densemicrostructure and has a lower porosity than the second layer.
 10. Themethod of claim 9, further comprising depositing a bond coat on thesubstrate, wherein the bond coat comprises at least one of Pt or MCrAlY,wherein M is Ni, Co, or NiCo, and wherein depositing the first pluralityof particles comprises depositing the first plurality of particles onthe bond coat.
 11. The method of claim 9, wherein the third plurality ofparticles further comprises alumina.
 12. The method of claim 9, furthercomprising depositing a fourth plurality of particles on the third layerto form a fourth layer, wherein the fourth plurality of particlescomprises a fourth base oxide comprising zirconia or hafnia, alumina,the second rare earth oxide comprising ytterbia, the third rare earthoxide, and a sixth rare earth oxide comprising at least one of lutetia,scandia, ceria, neodymia, europia, and gadolinia, wherein the fourthlayer comprises at least one of a dense microstructure or a columnarmicrostructure.
 13. The method of claim 9, wherein at least one of thefirst, second, or third plurality of particles defines an averagediameter of about 1 μm or less.
 14. The method of claim 9, wherein thesecond plurality of particles comprises pre-alloyed particles, whereinthe pre-alloyed particles comprise a cubic phase constitution.
 15. Themethod of claim 9, wherein the second plurality of particles comprisesfirst particles comprising zirconia, second particles comprisingytterbia, third particles comprising samaria, and fourth particlescomprising gadolinia.
 16. The method of claim 15, wherein oncedeposited, the second layer comprises a cubic phase constitution. 17.The method of claim 9, wherein the third plurality of particlescomprises pre-alloyed particles, wherein the pre-alloyed particlescomprise a tetragonal prime phase constitution.
 18. The method of claim9, wherein the third plurality of particles comprises first particlescomprising zirconia, second particles comprising ytterbia, thirdparticles comprising samaria, and fourth particles comprising gadolinia.19. The method of claim 18, wherein once deposited, the third layercomprises a tetragonal prime phase constitution.
 20. The method of claim9, wherein depositing the second plurality of particles or depositingthe third plurality of particles comprises: forming a suspensionincluding a respective set of particles and a solvent; introducing thesuspension into a heating plume of a plasma spray device; directing therespective set of particles in the suspension toward the substrate withthe heating plume; and depositing the respective set of particles toform a layer.